This invention relates to completing subsea wellheads.
Christmas trees having a horizontal production outlet offer several advantages for the production of oil and gas from subsea wellheads and have been in commercial use now for about 10 years. However, difficulties still arise from time to time in the use of such trees in orienting the tubing hanger so that the horizontal outlet in the tubing hanger is in alignment with the horizontal outlet though the body of the christmas tree. It will be appreciated that such problems typically arise far beneath the ocean's surface, where human intervention or even diagnosis is difficult.
Horizontal drilling techniques have advanced rapidly during the same period and have exacerbated the problem. Where the subsea well has a deviated wellbore, a great deal of force is required to twist the tubing hanger in the body of the christmas tree to bring the outlets into alignment, because the production tubing depending from the tubing hanger must undergo bending and twisting against the curvature of the wellbore and frictional contact between the production tubing and the casing tubing downhole in order for the tubing hanger to be rotated.
A mechanism commonly used to provide the required rotational force is a helix ramp situated near the bottom end of the christmas tree body and a key which is attached beneath the tubing hanger and follows the helix ramp as the tubing hanger is lowered into position in the christmas tree body. However, as discussed above, with the advent of directional drilling, the amount of rotational force and torque required to move the tubing downhole is greatly increased. A helix and key arrangement precision designed to better withstand this increased torque would be greatly appreciated.